1. Field of the Invention
The present invention relates to an optical recording medium on which data is written and read by means of laser light.
2. Prior Art
A variety of media are conventionally available for recording data with laser light, and among them is a medium whose recording layer is irradiated with laser light for locally heating the recording layer and causing a physical or chemical change such as melting, evaporation, sublimation or decomposition on an irradiated portion, i.e., for forming pits to record data.
The thin layer as the recording layer of the above optical recording medium is generally formed from any one of metals such as As, Te, Se and Ti or an alloy of at least two of them. However, the optical recording medium using the thin layer of any one of the above metals or the above alloy as a recording layer has the following defect. Since the recording layer has high thermal conductivity, long and narrow pits expand too wide in the width direction when formed for recording, and a pit-length modulation method for high-density recording cannot be applied.
On the other hand, for example, U.S. Pat. Nos. 4,622,179, 4,725,525, 4,492,750 or 4,458,004 discloses an optical recording medium whose recording layer is formed of an organic dyestuff having low thermal conductivity for applying the pit-length modulation method. However, the recording media disclosed in the above U.S. Patents are not yet satisfactory since the proposed organic dyestuffs still fax I to give a recording layer having excel lent physical and chemical stability.
Further, JP-A-2-42652 or JP-A-2-147,286 discloses an optical disc obtained by consecutively laminating a transparent substrate, a recording layer formed of an organic dyestuff, a reflection film and a protection film as an optical recording medium compatible with CD or CD-ROM on which data can be recorded by an eight-to-fourteen modulation (EFM) method based on pit-length modulation.
Cyanine compounds have been studied as the organic dyestuff for forming the recording layer of the above optical disc, and some of them have been put to practical use. In general, however, the cyanine compounds have poor light resistance and may cause a problem on recording reliability, and optical recording media using recording layer formed of cyanine compounds do not always have the device life equivalent to that of usual CD or CD-ROM.